Polypropylene is a very large volume product (8,013,000 metric tons in the U.S. in 2003). One of the major problems with its use is that there are no polar functional groups in the polymer to provide reactive sites. Such sites would be useful to allow adhesion, compatibility, paintability, crosslinking, etc. A typical way to solve this problem is to add in a small amount of material which contains polar groups attached to a low molecular weight polypropylene backbone. The polypropylene backbone bonds to the bulk polypropylene and the polar groups allow reactivity with external materials. The most common polar group that is incorporated is maleic anhydride. Typical of this approach is Eastman® E43, Eastman® AP550, or Arkema Orevac® grafted polypropylenes. A major problem with these materials is that they contain relatively low amounts of functional groups. The amount of maleic anhydride is determined by its acid number or SAP number. This is defined as the number of milligrams of KOH required to neutralize 1 g of the polymer. The polymers mentioned above have acid numbers of about 40–45. This corresponds to approximately 4 wt % of the material being from maleic anhydride.
It is difficult to increase the amount of maleic anhydride. The maleic anhydride is typically attached by grafting initiated by a radical chain process. It has been found that polypropylene undergoes chain scission via radical processes (note C. Tzoganakis, et al., “Controlled Degradation of Polypropylene”, Chem. Eng. Prog. 1988B, November, page 47, et seq. and D. Suwanda, et al., “Reactive Extrusion of Polypropylene II: Degradation Kinetic Modeling”, J. Appl. Polym. Sci. 1988b, 35, page 1033, et seq.) This is shown in the schematic sequence below:

The chain scission results in significant loss of physical properties due to decreases in the molecular weight of the polypropylene and consequent decreases in viscosity.
It is also known (see: B. C. Trivedi, et al. Maleic Anhydride, Plenum Press 1982 and “Maleic Anhydride, Maleic Acid, and Fumaric Acid” in Kirk-Othmer) that olefins will form alternating polymers with maleic anhydride in the presence of radicals. This has been shown to happen for polypropylenes prepared via metallocene catalysts. This happens because metallocene catalysts produce polypropylenes that have a single vinylidene group on the end of each molecule. Such polymerization is schematically shown below:
As can be seen from the end result shown above where polypropylene chains are joined by succinic anhydride moieties, the molecular weight greatly increases as does the viscosity of the material.
As noted, the incorporation of polar groups, such as maleic anhydride, into polymer can impart desirable properties to such polymers, e.g. having improved compatibility with polar compositions. For Example, U.S. Pat. No. 6,437,049 to Bortolon, et al., discloses a modified polypropylene prepared by means of a grafting reaction with maleic anhydride that can be advantageously used as a compatibilizing agent in the preparation of polypropylene reinforced with glass fibers or mixed with polyamide. In Bortolon, the polypropylene is reacted with maleic anhydride in the presence of dilauryl peroxide that functions as free radical initiator. This reaction is not problem-free as in an example where 10 percent by weight of maleic anhydride is reacted with a polypropylene, the resultant polypropylene included less than 4 percent by weight of succinic acid moieties.
U.S. Pat. No. 6,153,701 to Potnis, et al, discloses preparing a wettable polypropylene composition comprising polypropylene modified with maleic anhydride, and lists a number of documents that include maleic anhydride modified polypropylene.
There remains a need for polypropylene having increased succinic anhydride content and a method for producing such material.